Introduction

This article is based on the real task from the project I participated in, the task was to create a software system for protecting the Windows Clipboard against unauthorized data exchange. Though the Clipboard is one of the fundamental parts of the Windows operating system, there is little information about how it works, especially in the low level. In this article, I'm going to tell you something about the Clipboard internals by showing how you can forbid access to it. I simplified the task as much as it was possible to give you a possibility to focus on the most important parts.

The task

Create a program to forbid copying data via the Clipboard from one specified application (Wordpad) to another (Notepad). The names of the applications will be hardcoded. There will be no additional control possibilities. Also, no GUI is intended.

Who is this article for

The article is written for C/C++ developers, who are familiar with Windows API and kernel-mode driver development and who are interested in how to work with the Clipboard on the low level.

The reader is supposed to have some knowledge in C programming language, Windows API (both user- and kernel-mode ones) and the OS architecture. The reader doesn't have to be an expert in driver development but he/she has to know what drivers are and how to write the simplest one.

About the Windows Clipboard

Clipboard overview

As you know, the Clipboard is Windows component which enables applications to transfer data. An application places data into the clipboard for cut and copy operations and retrieves data from the clipboard for paste operations. Data in the clipboard can be in various formats. Each format is identified by an unsigned integer value.

An application places data into the clipboard in as many clipboard formats as possible, ordered from the most descriptive clipboard format to the least descriptive. For each format, the application calls the SetClipboardData function, specifying the format identifier and a global memory handle.

HANDLE SetClipboardData(
UINT uFormat,
HANDLE hMem
);

To retrieve data from the clipboard, an application first determines the clipboard format to retrieve. Typically, an application enumerates the available clipboard formats by using the EnumClipboardFormats function and uses the first format it recognizes. Alternatively, an application can use the GetPriorityClipboardFormat function. This function identifies the best available clipboard format according to the specified priority. An application can determine whether a format is available by using the IsClipboardFormatAvailable function.

After determining the clipboard format to use, an application calls the GetClipboardData function. This function returns the handle to a global memory object containing data in the specified format.

HANDLE GetClipboardData(
UINT uFormat
);

Before doing some operations with the Clipboard, an application must open it with OpenClipboard function. After finishing all operations with the Clipboard it must be closed with CloseClipboard.

How does it all work?

The Clipboard is implemented as a part of the Windows subsystem. API functions of this subsystem, including Clipboard API, are in user32.dll dynamic library. But the main work is preformed in win32k.sys module, which is the kernel mode part of the Windows subsystem. Win32k.sys is responsible for many things like managing windows and window messages, drawing graphics, working with the clipboard, etc. Win32k.sys has more than 600 functions which are undocumented and not exported.

Before placing data into the Clipboard application allocates memory using GlobalAlloc function and copies the data to that memory. Eventually, application calls SetClipboard with a format code and a handle returned by GlobalAlloc as arguments. SetClipboardData in its turn makes call to the kernel-mode function NtUserSetClipboardData, which performes actual work.

When some other application wants to get data from the clipboard it calls GetClipboardData, which also calls the kernel-mode function inside itself, NtUserGetClipboardData. If retrieving the data is succeeded, it returns the handle of the data.

Solution description

Possible task solutions

Now, let's get back to the task. We want to forbid copying data from one specific process to another. Copying some data via the Clipboard is actually separated on two independent operations: copy (or cut), which is represented by SetClipboardData and paste, represented by GetClipboardData. So, on copying we have to save the process name from which data is copied and on pasting we can choose, to allow or to forbid the copying.

There are two approaches. The first approach is to hook SetClipboardData and GetClipboardData in the user mode. Both functions are well documented and, that is much more important, they are exported. This is the main advantage. But this approach requires setting hooks for all processes in the system. Also we need to track creation of the new processes to set hooks in their address spaces too.

The second approach, which is described in this article, is to hook kernel-mode functions NtUserSetClipboardData and NtUserGetClipboardData. In this case we need to set hooks only once. But both functions are not exported from win32k.sys, so it is quite a problem to find them.

Project implementation

How to find an unexported function?

Hooking a couple of functions is not a big problem. But before that we need to find them. How to find a function which is not exported? One way is to hardcode the function address in the program. The address can be easily found manually, using WinDbg, for instance. Unfortunately, it is necessary to keep a base of these addresses for each operating system (and, probably, for each service pack) which you're going to support.

Other way is to keep not an address but some sequence of bytes from which that address can be obtained. This sequence of bytes is called unique signature. It can be a piece of the function's code or a piece of code from where the function is called. Though this way is not very portable too, it much less depends on differences between the various operating system versions.

Finding the signatures

First of all, we need to find the unique signatures for two functions: NtUserSetClipboardData and NtUserGetClipboardData. Both of them are in the win32k.sys. To do it, we will use WinDbg. Let's start with the signature for NtUserGetClipboardData.

As I mentioned the function we're looking for is in the win32k.sys and this module is mapped to a session address space, it means that it is available only in the context of some interactive process, like explorer.exe. To switch to explorer.exe, you can use !process 0 0 explorer.exe to get the address of its EPROCESS and then use .process command.

Now, look at the piece in the middle of the function code, where xxxGetClipboardData is called. The first function argument is copied to a local variable, then the values of the three registers are placed into the stack and finally the xxxGetClipboardData is called. That piece of code seems to be suitable for using as the unique signature. On the one hand, it can be unique (actually, it is) and on the other hand, we can hope that it can be used not only for this version of Windows.

The last byte of the second instruction is an offset of a local variable which can be easily changed in another build. So, it won't be included in the signature, as well as the last byte of the third instruction.

After checking some other versions of Windows we decide to use the described sequence of bytes as the unique signature for the NtUserGetClipboardData function. Unique signature for the NtUserSetClipboardData can be found in the same way.

Function search implementation

Signatures of the functions are pieces of binary code inside them. Therefore, the process of finding a function address consists of two steps: search forward for the signature and search back for the start bytes of the function.

Forbidding the Clipboard for the specified process

Because signatures may have "holes" (insignificant bits), it is necessary to implement comparison with mask, to compare only significant parts. It is implemented in function PlCompareSequence. PlSearchSequence contains implementation of the forward search using comparison with mask.

Structure MASK_BUFFER represents buffer which may have insignificant bits. It is represented by pointer to data, pointer to mask and length of both data and mask. This structure is used for storing signatures. Structure BUFFER represents plain buffer in memory, described by pointer to buffer and its length.

Function PlGetProcAddress is used for searching functions by signature. Firstly, it searches for the specified signature. If a match for the signature is found, it checks, is there are another matches for it. If there is only one match for the signature, then back search for the start bytes is performed.

Setting hooks

To hook the functions we'll use a very common technique, so if it is familiar to you, you can just skip this part of the article.

The technique implies replacing the first bytes of a target function with a jump to our hook function, which has the same prototype. First bytes of the target function are to be copied to somewhere to keep the possibility to use the function. Hence, when user calls the target function he gets to the hook function.

But there is one thing to remember. You cannot divide an instruction into parts, so only whole instructions should be copied. It makes us to detect lengths of instructions we move. Also, if there are some relative addresses, they must be corrected after moving. To detect instruction length and relative addresses we'll use slightly modified Hacker Disassembler Engine, a small library by Veacheslav Patkov which suits well for the task.

To store names of the source and destination processes, structure CLIPBOARD_CONTEXT is used. When NtUserSetClipboardData is called we save name of the current process in the usFrom field. When NtUserGetClipboardData is called, the name of the current process is stored in the usTo field. If both namesB match the forbidding rule, we return NULL to caller, which indicates an error.

Building the sample

To build the code sample, you need to install Windows Driver Kit. Also you have to set an environment variable BASEDIR to the path where you have installed WDK.

After that you can just use Visual Studio to build the sample.

Supported Windows versions

The project was tested on the following versions of Windows:

Windows XP (SP2 and SP3)

Windows 2003 Server

Windows Vista (SP1)

Conclusions

In the article I described how to forbid the Clipboard for the specified process from a kernel-mode driver.B Because of the code sample in this article is made rather for educational purposes, it has some limitations. First of all, the sample works only on x86 operating systems. It also has never been tested on Windows 2000 and Windows 7. The sample may work incorrectly in a Remote Desktop session.

There is also one thing about forbidding copying data from the some process to itself. Some applications, like Microsoft Excel, can detect when data is copied and pasted within one process. If so, such applications may not use Clipboard API at all, therefore it makes some problems when you want to forbid it.

About the Author

Apriorit Inc.

ApriorIT is a Software Research and Development company that works in advanced knowledge-intensive scopes.
Company offers integrated research&development services for the software projects in such directions as Corporate Security, Remote Control, Mobile Development, Embedded Systems, Virtualization, Drivers and others.
Official site http://www.apriorit.com

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